Most conventional mechanisms fitted on milling machines are awkward to use; the problem is that fixing and removing the tools necessitates the unscrewing of a threaded rod at an upper extremity of the milling machine. This operation, which often requires the use of a stepladder, involves a considerable waste of time, as numerous tools have to be used sequentially.
There exist a number of devices which are an improvement over the conventional system, but none of them are free from drawbacks. In all of them, centering is effected by means of a detachable cone sleeve, which thus extends the nose of the spindle by two to five centimeters. This extension lessens its machining capacity and, consequently, its capacity to work on large-size parts. The lengthening also creates an imbalance with consequent loss of rigidity and occurrance of vibrations, especially in the case of any heavy stress exerted on the machine; moreover there are sometimes difficulties in centering milling spindles on account of the way they are designed. These drawbacks are due to the difficulty of perfectly cleaning and maintaining the spindle cone, or the cone sleeve.